Ovicidal and Adulticidal Activities ofOriganum majoranaEssential Oil Constituents against Insecticide-Susceptible and Pyrethroid/Malathion-ResistantPediculus humanus capitis(Anoplura: Pediculidae)

The toxicity of essential oil constituents from marjoram, Origanum majorana, to eggs and adult females of the susceptible KR-HL and dual malathion- and permethrin-resistant BR-HL strains of human head louse, Pediculus humanus capitis, was examined using contact + fumigant mortality bioassay. Results were compared with those following treatment with two pyrethroid pediculicides, d-phenothrin or pyrethrum. As judged by the lethal time to 50% mortality (LT50) values at the exposure rate of 0.25 mg/cm2, 1,8-cineole (14.1 min) was the most toxic compound, followed by linalool (15.4 min) to KR-HL females. These compounds were faster acting than either d-phenothrin (24.1 min) or pyrethrum (33.4 min). Based on the lethal concentration causing 50% mortality (LC50) values, (−)-camphor (0.022 mg/cm2) was the most toxic compound, followed by linalool (0.035 mg/cm2), (−)-terpinen-4-ol (0.040 mg/cm2), α-terpineol (0.045 mg/cm2), and 1,8-cineole (0.068 mg/cm2) against KR-HL females. These monoterpenoids were less toxic than either d-phenothrin (LC50, 0.0015 mg/cm2) or pyrethrum (0.0013 mg/cm2). However, the toxicities of these monoterpenoids were almost identical against females from either of the two strains, even though the BR-HL females exhibited high levels of resistance to d-phenothrin [resistance ratio (RR), 667] and pyrethrum (RR, 754). After a 24 h exposure to linalool, BR-HL egg hatch was inhibited 100 and 84% at 0.25 or 0.125 mg/cm2, respectively, while (−)-terpinen-4-ol caused 94 and 69% inhibition of egg hatch at 0.25 and 0.125 mg/cm2. α-Terpineol caused 88 and 76% inhibition of egg hatch at 0.5 and 0.25 mg/cm2, respectively. Thus, certain monoterpenoids from O. majorana essential oil, particularly linalool, (−)-terpinen-4-ol and α-terpineol, merit further study as potential pediculicides and ovicides for the control of insecticide-resistant P. h. capitis populations as fumigants with contact action.